The present invention relates to an image forming apparatus such as a printer, a facsimile, a copier, and a multifunction product. More specifically, the present invention relates to an image forming apparatus having a fixing device with a heater.
In a conventional image forming apparatus, after a developer image as a toner image is formed on a photosensitive drum, a fixing roller and the like of a fixing device fixes the developer image at a specific temperature. The fixing roller is heated to a specific temperature with a heater that heats with an electrical current flowing from an alternate current power source. The alternate current power source for the heater to heat tends to cause a problem such as a high frequency current or a shortened life of a heating device.
To this end, there has been a technology, in which a zero cross circuit is provided for detecting a zero cross point of an alternate current power source voltage, and a phototriac coupler is operated to perform on-off control per every half cycle based on the zero cross point. Accordingly, it is possible to prevent the alternate electrical current supplied to the heater from shifting to a positive side or a negative side. As a result, it is possible to suppress the high frequency current and extend a life of the heating device.
Patent Reference has disclosed a conventional image forming apparatus. In the conventional image forming apparatus, a central process unit (referred to as a CPU) is provided for transmitting a heating off signal, so that an alternate electrical current supplied to the heating device is controlled and shut off. When the CPU transmits the heating off signal, the CPU generates a control signal to regularly turn on and off an alternate current voltage per half cycle for a specific number of times. A switch element is provided for applying the alternate current voltage to the heating device according to the control signal.
Patent Reference: Japanese Patent Publication No. 10-213996
In the conventional image forming apparatus, a zero cross circuit is provided for detecting the zero cross point. According to the zero cross point thus detected, a heater conduction timing is synchronized with the zero cross point, so that the heater is conducted per half cycle. Accordingly, it is necessary to provide the heater conduction circuit with a complex configuration.
In view of the problems described above, an object of the present invention is to provide an image forming apparatus capable of solving the problems of the conventional image forming apparatus.
Further objects and advantages of the invention will be apparent from the following description of the invention.